1. Field of the Invention
One embodiment of the present invention relates to a voltage controlled oscillator that can control the oscillation frequency of an output signal in accordance with the voltage of an input signal and a semiconductor device including the voltage controlled oscillator.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
A voltage controlled oscillator (VCO) generally includes a ring oscillator including a plurality of delay elements and a current source controlling the amount of current supplied to the delay element in accordance with voltage. In the voltage controlled oscillator, the oscillation frequency of an output signal is controlled by changing the delay time of a signal in the delay element in accordance with the amount of current supplied from the current source to the delay element.
Patent Document 1 discloses a voltage controlled oscillator in which a delay element including a depletion MOS transistor at an output end of an inverter is used (FIG. 6) and the delay time of rise or fall in an output signal of the inverter is controlled in accordance with the on resistance of the MOS transistor.